In general, mechanisms such as a transmission, a clutch and so on are arranged on a power transmission route of a vehicle from a prime mover to a wheel, and those mechanisms are actuated by a hydraulic control unit. One example of this kind of hydraulic control unit is disclosed in Japanese Patent Laid-Open No. 2004-353694.
The hydraulic control unit taught by Japanese Patent Laid-Open No. 2004-353694 comprises a plurality of oil pumps including a main pump and a sub pump driven by at least one of an engine and a motor. According to the teachings of Japanese Patent Laid-Open No. 2004-353694, an oil discharging outlet of the main pump is connected with a first oil requiring portion thorough a first oil passage. A primary regulator valve is arranged on a route from the first oil passage to a second oil passage connected with a second oil requiring portion. Further, a secondary regulator valve is arranged on a route from the second oil passage to a third oil passage. An oil discharging outlet of the sub oil pump is divided into two directions, and one of the oil discharging outlets of the sub oil pump is connected with the first oil passage through a first check valve. The other oil discharging outlet of the sub oil pump is connected with a changeover valve, and the changeover valve is connected with the secondary regulator valve. The other oil discharging outlet of the sub oil pump can also be connected with the second oil passage through a second check valve. The third oil passage is connected with the main pump and a suction inlet of the sub oil pump.
Operating oil discharged from the main oil pump is fed to the first oil requiring portion through the first oil passage. In case oil pressure in the first oil passage is raised, an amount of the operating oil discharged from the first oil passage to the second oil passage through the primary regulator valve is increased. That is, the oil pressure in the first oil passage is controlled by an action of the primary regulator valve. On the other hand, the operating oil in the second oil passage is fed to the second oil requiring portion. In case the oil pressure in the second oil passage is raised, an amount of the operating oil discharged from the second oil passage to the third oil passage is increased.
In case oil presser in the oil discharging outlet of the sub oil pump is higher than that in the first oil passage, the first check valve is opened to feed the operating oil discharged from the sub oil pump to the first oil passage. To the contrary, in case the oil presser in the oil discharging outlet of the sub oil pump is lower than that in the first oil passage, the first check valve is closed. When the changeover valve is opened, the oil discharged from the sub oil pump is fed to an inlet of the second check valve through the changeover valve. Here, in case the oil pressure in the second oil passage is relatively low, the secondary regulator valve is closed and the second check valve is opened. As a result, the operating oil discharged from the sub oil pump is fed to the second oil passage. To the contrary, in case the oil pressure in the second oil passage is relatively high, the secondary regulator valve is opened, and the operating oil in the second oil passage is discharged to the third oil passage. When the primary regulator valve is opened, the operating oil discharged from the sub oil pump is fed to the third oil passage through the changeover valve and the secondary regulator valve. According to the configuration thus far explained, the oil pressure in the second oil passage is lower than that in the first oil passage, and the oil pressure in the third oil passage is lower than that in the second oil passage. Thus, according to the hydraulic control unit taught by Japanese Patent Laid-Open No. 2004-353694, the primary regulator valve is opened and closed according to a change in the oil pressure in the second oil passage, and the oil discharging outlet of the sub oil pump is connected selectively with the second and the third oil passages.
According to the hydraulic control unit taught by Japanese Patent Laid-Open No. 2004-353694, however, in case a property determining a relation between the oil pressure in the second oil passage and the amount of the oil required by the second oil requiring portion is changed, the secondary regulator valve is closed if the oil pressure in the second oil passage is low, and the operating oil discharged from the sub oil pump is thereby further fed to the second oil passage, in spite of the amount of the oil in the second oil requiring portion is sufficient. As a result, a drive loss of the sub oil pump may become worse.